Method and circuit for compensating for non-uniformities in display storage tubes



Dec. 24, 1963 M. scoTT ETAL 3,115,592

H. METHOD AND CIRCUIT FOR COMPENSATING FOR NON-UNIFORMITIES IN` DISPLAY STORAGE TUBES Filed 0G11. 4. 1960 K 5 Sheets-Sheet l Y Ham( -fdn/ IN V EN TORJ Hnward M. Scott Claude E. Reeder .Bq 77W W7,

Amma;

Dec. 24, 1963 H. M. SCOTT ETAL 3,115,592

METHOD AND CIRCUIT FOR OOMPENSATING FOR NoN-UNIFORMITIES IN DISPLAY STORAGE: TUBES Filed 001;. 4. 1960 3 Sheets-Sheet 2 Howard Smit Claude E. Rev

Atwmeq Dec 24, 1963 H. M. scoTT ETAL 3,115,592

METHOD AND CIRCUIT FOR COMPENSATING FOR NoN-UNIFORMITIES IN DISPLAY STORAGE TUBES Filed Oct. 4, 1960 3 Sheets-Sheet 3 I 77Mi- INVENTORS H m d mswit @12u32 E, Reeer By Amman llS-,SQZ Patented Dec. 24, i963 3,l15,592 AND CERQUTT FR CMFENSTENG FR NN-UNEFRVHTES HN DESPLAY STRAGE TUBES Howard M. Scott, Philadelphia, Pa., and Claude E.

Reeder, Pennsauken, N5., assigner-s to Radio Corporatien ai America, a corporation of Delaware Filed Get. d, 196i?, Ser. No. 60,370 9 (Ilairns. ('Ci. SiS-ft2) This invention relates to signal storage and display and particularly to improved methods and means for utilizing a display type electrical storage tube for displaying signal intelligence.

In display storage tubes of the general type shown in Knoll Patent 2,856,559 and in Hook Patent 2,843,798 a picture is obtained by causing a focused writing beam to produce a charge pattern on a storage screen. The writing beam is intensity modulated as it scans the storage screen, thus laying down the charge pattern.

The storage tube also includes a viewing gun that directs a spray of electrons (a viewing beam) toward the storage screen. Behind the storage screen is a viewing screen such as a phosphor screen. The tube structure also includes a collector screen on the beam side of the storage screen which is spaced close to the storage screen.

Electrons of the viewing beam pass through the interstices of the storage screen as a function of the charge pattern laid down on the storage screen, and strike the phosphor screen to produce a picture thereon that corresponds to the charge pattern.

in manufacturing storage tubes of this type it is difficult to produce tubes that do not have non-uniformities such as slight variations in spacing between the electrodes, especially the collector screen, the storage screen, and the phosphor screen. Still other non-uniformities may result from the collimator electrodes of the tube and the density of the viewing beam. Also, the angle at which the writing beam strikes the storage grid may reduce the amount of the stored charge because at the large angles the collector screen may absorb some of the writing beam energy.

As a result of such variations in uniformity, the picture appearing on the phosphor screen of a display storage tube may have a non-uniform background. For example, the background may contain light and dark areas or spots, or one half of the picture may have a darker background than the other half, or there may be light and dark rings. ln addition to the fact that such a picture is not pleasing to the observer, some of the picture information may be lost because of poor halftone display at the very low light levels.

An object of the invention is to provide improved methods of and means for reducing the above-described adverse effects on the picture of a display storage tube.

A further obiect of the invention is to provide methods of and means for producing an improved picture on a display storage tube.

A further object of the invention is to provide an improved signal display system.

A still further object of the invention is to provide improved methods of operating a display storage tube.

In practicing one embodiment of the invention, a small amplitude sine wave or other repetitive voltage, referred to as a compensating voltage, and having a 400 cycle frequency, for example, is applied between the storage screen and, for example, the cathode of the viewing gun of the tube. As a result, formerly black screen areas are periodically brightened so that a picture of improved background is obtained. Also, as will be explained hereinafter, certain picture information that formerly was obscured is displayed in an improved manner.

The invention will be described in detail with reference to the accompanying drawing in which:

FlG. l is a cross-sectional schematic diagram, partially in block form, of a display storage tube system embodying the invention;

FTG. 2 is a schematic diagram, in block form, of a modification of the system shown in FIG. 1;

FlG. 3 is a schematic circuit diagram illustrating a modification of the system shown in FIG. l;

FlG. 4 is a group of graphs which are referred to in explaining the invention; and

FTG. 5 is a group of graphs corresponding to a portion of FlG. 4 on an enlarged scale.

Similar reference characters are applied to similar elements throughout the drawing.

For an understanding of the invention one should first understand the construction and operation of display storage tubes. The description which immediately follows under the headings Storage Tube Structure, Tube Operation and Erasing is for the purpose of providing this preliminary understanding of display storage tubes and their operation.

The later section under the heading Compensation for Providing Improved Halftone Response and for Providing More Uniform Background relates speciically to the present invention as applied to display storage tubes.

Storage Tube Structure FIG. 1 of the drawing shows one example of a display type storage tube consisting of an evacuated envelope it) having two neck sections )l2 and i4, respectively. Within the envelope neck l2 is an electron gun i6, hereinafter referred to as the viewing gun. Within neck 14 is a second or writing electron gun 18 for providing a modulated beam of electrons which is accelerated into the envelope portion l0.

Mounted at the large end of the envelope portion 10 is a phosphor screen assembly 2d including a glass support sheet 22 having a thin conductive film 2d disposed on one surface thereof and facing the electron guns. The film 24 may be formed, for example, of a metal or metallic compound such as tin oxide. On top of the conductive film 2d is a layer 26 formed from a phosphor material which luoresces under electron bombardment.

ln the direction towards the electron guns from the surface of the uorescent layer 26 is a storage screen Z7 which includes a line mesh metal screen 28 spaced at a distance of two or more millimeters, for example, from the fluorescent layer 26. The storage screen 27 further includes capacitive or storage elements 3@ on the metal screen Z8.

The capacitive elements E@ may be formed, for example, by evaporating on the surface of the metal screen 28 a dielectric insulating material such as silica or niagnesium fluoride to form thereon a lrn of the order of several microns in thickness. The metal screen 2S is referred to as the backing-electrode of the storage screen 27. A Second line mesh metal screen 32, which is referred to as the collector screen, is located a few millimeters frorn the storage screen 27 in the direction towards the electron guns. Screen 32 may be a woven stainless steel screen of the order of 230 lines per inch while the conductive screen 2S and the storage screen 27 may have a fineness of the order of ZO lines per inch.

The phosphor screen assembly 20 and the storage screen 27 `are mounted on an annular metal support ring llt?. The ring rtl) supports intermediate its ends the glass support sheet 22 and across its open end the conductive screen 2.8. The support ning 4d is mounted on a ring 36 of insulating material. The ring 3d is iixed within the envelope by supporting means not shown. Also mounted on the insulating ring 36 is a second annular metal support ring 42 across the end of which is mounted the woven metal mesh screen 32. The conductive tin oxide iilrn 24 is insulated from the support ring `42 by the glass sheet 22 and is connected by a lead 44 to a source of positive potential outside the envelope 16. Mesh screen 32 also is connected to a source of positive potential via lead 416. rIlhc conductive screen 23 of the storage screen during the tube operation, is set either at ground potential or a bias potential for writing or at a more positive potential of the order of twenty volts, for example, for erasure, as will be shown hereinafter.

The viewing gun 116 comprises a cathode electrode 48, a control electrode 5t), a first accelerating electrode 52, and a second accelerating electrode 54 mounted ysuccessively along the axis of the gun 16 toward the face plate 38. During the tube operation these electrodes are maintained at appropriate voltages to form the electron emission from the cathode 48 into a wide beam or spray 56 of electrons Iwhich is referred to as the viewing beam. The inner surface of the envelope llt? has applied thereto a conductive coating S3 of colloidal graphite which coating may be maintained at the same positive potential as the second accelerating electrode 54. A second wall coating 6() extends from a point spaced from but adjacent coating 5S over the bulb wall enclosing the assemblies 2li and 27. This coating is at a potential different from that of coating "3 and thus provides a Collimating electron lens to align the electrons of the spray beam 5S in a direction axially with respect to the target assemblies.

The writing electron gun 18 comprises a cathode electrode 62;, a control electrode 64 and, successively spaced ltoward the target, a irst accelerating electrode 66 and a second accelerating elect-rode 63. The wall coating 5S extends into the neck 14 of the writing gun and forms a third accelerating electrode for forming the electrons of gun 18 into a sharply defined and focussed beam 74B.

The voltages indicated in FIG. l as applied to the electrodes of the above tube are illustrative of typical suitable operating voltages but should not be considered as limiting.

Tube Operation To prepare the storage target for storing a charge pattern on the capacitive elements 3G of the storage screen 27, it is necessary to establish a uniform potential thereover. With the viewing gun turned on, the electrons of the spray beam 56 are accelerated with energies up to 1,000 volts through the metal mesh screen 32. rPhe potential of conductive screen 28 of the storage screen is set to a potential sutiiciently positive (of the order of 20 volts positive relative to ground, for example) that the electrons of the viewing beam 56 strike the storage surface 3@ of the storage screen at velocities or energies to initiate secondary emission from all portions of the surface. ln the present example, the positive potential or" 2t) volts, which may be an erase pulse, is applied, as described hereinafter, to the conductive screen 2S is below the first cross-over point on the secondary ratio curve of the silica (or magnesium fluoride) iilm. The rst cross-over point for a silica storage iilm is of the Iorder of 75 to 150 volts while the rst cross-over point for a magnesium uoride film is approximately to 60 volts. Thus secondary emission is initiated having a ratio less than unity and the storage screen surface 30 assumes a uniform potential, in this instance viewing gun cathode potential. The entire surface of the storage screen may vbe completely erased to a uniform potential, as described above, in a fraction of a second.

The potential `applied to the conductive screen 28 of the storage screen is then set to approximately ground potential. In some cases it may be desirable to apply either a small positive bias or a small negative bias to the screen 2S. Because of the thinness of the storage surface of the storage screen, the surface 3d is closely coupled capacitively to the backing screen 2S, hence the relative potential difference therebetween is maintained; i.e., as the potential of screen 2.8 is changed from a 20 volts positive relative to ground (as by erase pulses discussed hereinafter) to ground potential, the potential of screen surface 3@ changes by a corresponding amount from viewing gun cathode potential (ground potential) to `approximately minus 2() volts relative to ground, i.e., the screen surface 3o assumes a minus 2O volt bias. The electrons of the viewing beam 56 are accelerated through the collector screen 32 and enter a retarding field adjacent the screen surface 3i?, the retarding lield turning the elecrons back to the metal screen 32 which serves as a collector therefor. The 8,060 volt potential applied to the tin oxide iilm 24 of the phosphor screen creates a field which tends to extend through the interstices of the storage screen 27 to draw electrons through the screen to bombard the duorescent layer Z6. The voltage `to which the storage screen surface 3i? is set (minus 2O volts), however, just prevents any electrons from passing therethrough. This is referred to as the viewing beam cut-off voltage.

The writing gun 18 is then turned on an produces a sharply defined and focused beam 7d) which may `be deiected to scan over the storage surface 3d. The deliection may be accomplished, for example, by supplying vertical and horizontal pairs of dellection plates 72` and 74, respectively, with suitable deflection signals yfrom deflection generators W and 73, respectively. While theA writing beam 7G is being deliected in the desired pattern, the beam itl is modulated by video signals applied tov the writing gun control grid `64 from an input circuit 30..

The writing beam impinges on the storage surface 3@ at.

a voltage of `approximately 3,000 volts which is between the irst and second cross-over points on the secondary` emission ratio curve thereof.

In this manner the Writing beam initiates secondary emission from the surface of the storage screen such that, more electrons leave the surface than impinge thereon., ln those areas where the beam '7% strikes, the storage, elements 3) of the storage screen are driven positively'v from their potential of minus 20 volts toward viewing gun cathode potential or ground.

26 and cause luminescence.

ondary emission and hence corresponding to the image pattern of the writing beam.

This type of writing provides a visual display in which stored information appears as white on a darli background. Once a signal has been stored and displayed, theoretically it should remain stored and displayed indefinitely since the mode of tube operation described above is such that the low velocity viewing beam 56 normally does not come in contact with the charged areas of the storage surface and therefore does not disturb the established charge pattern.

Erasing FIG. 1 shows a suitable erasing system for use with the display storage tube. This system is adapted for Provided the ratioy of the viewing beam average current density to the aver-4 age current density of the writing beam over a given; time is unity or greater, no point on the storage surface- 3i) will stay positively charged with respect to groundl since electrons from the Viewing beam 55 land contin uously at that point and drive it back to ground potential, or slightly negative with respect to ground. In theareas where the storage surface 3d has been driven positively (from minus 20 volts) the positive field of thel phosphor screen 2e now penetrates to draw the low en orgy electrons of the viewing beam 56 through. the interstices of storage screen 27 to strike the phosphor layerI This luminescence appears: only on areas of the phosphor layer 26 corresponding toareas of the storage surface driven positively by sccthrough a coupling capacitor and through the diode 1454 to the higl. potential end of the load resistor 92. A positive bias from a battery itl@ is applied to the anode side of the diode ill so that it will pass the full compensating voltage. lf the compensating voltage is three volts, for example, the bias voltage may be two volts.

Since the erase pulse has a higher peak voltage than the peak-to-peak voltage of the compensating voltage wave, the diode is made non-conducting upon the occurrence of the positive potential erase pulse. Thus, the compensating voltage is never superimposed on the erase pulse.

The relation of the value of the compensating Voltage IZ to the pertinent characteristics or" a particular display storage tube may be understood by reference to FIG. 4.

The graphs A-A' and A--A are obtained from a particular display storage tube by plotting the brightness or luminosity of the phosphor screen against the voltage of the storage elements 3?; of the storage screen Z7. This will bc understood from the following description of how the graphs are obtained.

The writing bea-.rn is turned off and the viewing beam is turned on. Also, the oscillator Mill is turned off or connected. The storage screen is soon at an equilibrium condition and the phosphor screen is at a uniform white. The storage elements of the storage screen 2' have now been brought to the potential of the cathode of the viewing gun by the viewing beam electrons.

Next an erase pulse of comparatively small voltage is applied to the storage screen. The peak amplitude of the erase pulse voltage may be such as to reduce the screen brightness down to the point l on the graph, for example. The erase pulse amplitude is then increased to reduce the phosphor screen brightness still further to the point 2, for example.

It will be understood that the erase pulses of a selected peak amplitude are applied ror a period long enough to effect the maximum erase or brightness reducing effect that will be produced by pulses of the particular amplitude selected. It may be noted that the abscissa voltage representing the voltage of the storage elements 3i) of the storage screen is the same as the peak Voltage amplitude of the erase pulses. Thus, point l may be plotted from the amplitude of the applied erase pulse and from the resulting brightness of the phosphor screen.

As this plotting is continued by successively increasing the voltage of the erase pulse it will be found that some area or areas of the phosphor screen will become darker other areas. This is the non-uniform background effect due to the tube non-uniformities previously mentioned. For example, the graph A may continue along the graph section A. The graph A-A in the illustration is the graph for the screen area or spot that goes black first as the erase pulse is successively increased in amplitude for the plotting; it is the graph for the minimum viewing beam cut-olf. In the example, a spot on the screen has gone black at the point labeled cut-off point for darkest spot. The graph A-A may now be plotted for the lightest area or spot on the phosphor screen. This is the graph for the maximum viewing beam cut-off. It will be apparent that the storage screen had to be brought to a more negative Voltage to make this lightest area go black as indicated by the legend cutoff point for brightest spot.

The difference between these two cut-off voltages is indicated as AV. In the example illustrated in FIG. 4 the compensating voltage i162 applied to the storage screen has a peak-to-peak value of about two times AV.

T he operation of a display storage tube using the compensating voltage wave 102 will now be described with reference to FIG. 4. In the description it will be assumed that, as described in connection with FIG. 3, the erase pulse and the compensating voltage do not add. The peak amplitude of the erase pulse is set to a value such that it brings the storage elements E@ of the storage screen to an intermediate potential between the cuto potentials for the darkest spot and the lightest spot. In the example illustrated, this intermediate potential (the bias potential) is midway between the two cut-olf potentials. In order to simplify the description, an operation is assumed where complete erasure is effected at the end of a picture frame, i.e., during the frame return time.

At the beginning of the next picture fname, the erase pulse is olif and the storage elements 3i? of the storage screen have their potential changed with respect to ground in accordance with the applied compensating voltage wave IGZ. This is because :the capacity `elements Sti are disconnected for iloating since the electrons of the viewing beam no longer strike them, the enase pulse being of".

It is evident `from FIG. 4 that onehalf cycle of the voltage Wave MP2 swings the storage `elements 3@ of the storage screen less negative so that the darkest spot on the phosphor screen will become light, i.e., no longer black. It follow-s that the entire surface of the phosphor screen momentarily becomes light. This result is plotted in FIG. 4 which sho-ws that now the darkest spot area of Ithe screen periodically has the brightness indicated by the graphs i692 `It is apparent that some picture information that might have been lost without using the compensating voltage now is not lost, and that no spot on the screen will appear black.

A similar plotting with respect to the graph A-A for the lightest spot on the phosphor screen shows that this area of the phosphor screen has the brightness indicated by the graphs lill. As a result of the compensation, this erea may be slightly increased in effective brightness, it may be substantially unchanged, or it may be decreased in effective brightness, depending upon tube character* istics and adjustments.

It will be noted that the other half cycle of the compensating voltage causes the entire surface of the phosphor screen to become black momentarily. Thus, in the exemple illustrated the entire phosphor screen surfaces goes alternately light and black.

In the foregoing discussion it is assumed that the writing beam is not laying down a charge on the screen areas being considered, that is, cer-tain areas corresponding to the graphs A-A and A- During openation a writing beam, if modulated, will lay down charges on so-me or possibly all areas of the storage screen, but the effect of the compensating voltage is best illustrated by assuming that no charges are laid down on @at least one area of darkest spot and one area of lightest spot.

In addition to providing more picture information, the use of the compensating voltage makes the picture background less spotty, i.e., more uniform and less contrasty. it will be helpful to refer to FIG. 5 in Adiscussing how the picture background is made less contrasty. In FIG. 5 the graphs correspond -to those in FIG. 4 with the lower portions of the graphs A-A and A-A enlarged for more laccurate plotting.

A comparison of the background contrast Afor two condition-s will be made. The first condition is where the erased storage screen has no compensating voltage wave on it and is biased by the amount of the peak amplitude of the erase pulse. F or the idark screen `area (graph A) fthe light rlevel is black. For the brightest screen area (graph A) the light level is represented by line M2. The `amount of contrast is indicated by the double arrow H3.

The second condition is where Ithe erased storage screen has the compensating voltage 102 applied to it. For the `dark screen area (graph A) the light produced on the phosphor screen is represented by graph 169; The apparent brightness is the average brightness for the time the area is not black (represented by dotted rectangle llli) times the percentage of the complete cycle period that the 'area is light. This percentage in the illustration is about one-third. Therefore, the light level (apparent cyclically and incrementally erasing data stored and displayed thereby. A synchronizer 82 repetitively produces pulses at a predetermined pulse repetition rate. These pulses are fed to a horizontal detiection wave generator 7S and to a pulse rate divider circuit 84. The divider circuit produces one output pulse in response to a predetermined number of input pulses. Assuming that the erasing is to be utilized in connection with a typical B- scan radar system, the divider circuit 34 may produce one Output pulse in response to each 6,000 pulses input thereto. This assumes a synchronizer pulse repetition rate of 1,000 cycles per second and an azimuth scanning rate of one 360 Search every six seconds. Each output pulse derived from the divider 84 is applied to a vertical detlection wave generator 76. The horizontal and vertical deection generators 73 and '76, respectively, produce saw-tooth deliection signals which are applied to the pairs of deflection plates 72 and '74 of the storage tube to deect the writing beam 7@ to rectilinearly scan the storage surface 39. Y

During the horizontal deflection intervals, video signals are applied to the writing gun control electrode 6d via input circuit titl. They modulate the intensity of the writing beam 7i) to establish a predetermined charge pattern on the storage surface 3@ and a corresponding visual display on the phosphor layer 26.

Simultaneous with the application of synchronizer pulses to the divider circuit 84 and to the horizontal deflection generator 73, the synchronizer pulses also are successively applied to a time delay circuit Se and to a one-shot multivibrator 83. The time delay provided by the circuit S6 is adjusted such that the leading edges of the one-shot multivibrator pulses each occur slightly after the writing of each line of information. The multivibrator pulses thus produced are applied to an ampliiier gli, preferably a cathode-follower. The output pulses 39 (the erase pulses) of amplifier @il are supplied through a coupling capacitor 9i and an isolating resistor Q3 to a load impedance 92 connected between the conductive screen 2S and ground so as to periodically and instantaneously drive the conductive screen 2g approximately 2() volts positive with respect to ground potential to effect partial erasure of the stored data. By this means information stored in the display tube incrementally is erased in the flyback interval following each writing interval.

The erase interval may be controlled by controlling the multivibrator pulse duration and the extent or depth of erasure may be controlled by suitable adjustment of the amplitude of the erasing pulse. The erasing pulse duration may be controlled, for example, by varying circuit constants in the charging circuit of the one-shot multivibrator S3 while the erasing pulse amplitude may be controlled by means of a gain control circuit 9d associated with the amplifier 90. By properly adjusting the amplitude and duration of the erasing pulse, it will be seen that a given line of data is partially erased during each flyback interval and is completely erased in one frame time and just prior to being replaced with a line of new data. Thus an up-to-date display of information is provided wherein old data cyclically may be erased and replaced with new information.

Signal integration and noise discrimination may be aiforded by employing the system shown in FIG. 2 of the drawing. Signals applied to the input of the time delay circuit 86 are derived from the output circuit, rather than the input circuit, of the pulse rate divider circuit 34. Thus erasing pulses are produced only in the time intervals between the writing of successive frames of information.

in the mode of operation for obtaining signal integration, both desired signals and noise signals are Written during a given frame time and are partially erased just prior to the writing of the next frame of data. in the next frame time the desired (and partially erased) signals are rewritten to enhance the charge pattern established during the writing of the preceding frame. The noise signals, however, because of their random occurrence are not rewritten in the same areas on the tube storage screen 30 in successive frames. Thus the desired signals are integrated and noise signals are erased.

Another mode of operation for the system of FIG. 2 may be employed. In this mode of operation complete erasure at the end of each picture frame is effected, there being no signal integration in this case.

l'n the foregoing description, the erase pulse is described as synchronized with the deflection waves. It should be understood that such synchronization is not necessary, and that the present invention applies equally well to a storage tube system in which the erase pulses are not synchronized with the deflection.

Also, it may be noted that the peak amplitude of the erase pulse depends upon the particular storage tube construction. Thus, instead of an erase pulse of 20 volts as assumed in the example, one storage tube may be operated with a 5 volt erase pulse, and another tube with a l2 volt erase pulse.

Compensation for Providing Improved Halftone Response and for Providing More Uniform Background In accordance with the present invention the potential of the storage screen is varied repetitively woth respect to the cathode of the viewing gun. This may be done, as shown in FlG. l, by means of an oscillator i491 which has one terminal connected to ground (the cathode of the viewing gun is also connected to ground), and which has the other terminal connected to the backing-electrode 2S of the storage screen 27 through a coupling capacitor 95' and an isolating resistor 97. The oscillator ll may be a sine wave oscillator or it may be one that supplies a non-sinusoidal Wave. For example, it may be a multivibrator that supplies a square wave, or an oscillator that that supplies a sawtooth or triangular wave.

The frequency of the compensating voltage lltlZ supplied by oscillator 101 preferably is at least equal to that corresponding to the periodV for persistence of vision. The frequency otherwise is largely a matter of convenience. Examples of satisfactory frequencies are 60 cycles per second, 40() cycles per second, and 10,000 cycles per second.

The proper pealt-to-peak value of the compensating voltage supplied by oscillator lill depends upon the particular storage tube. For some tubes one volt peak-topeak is satisfactory. For other storage tubes six volts peak-to-peak may be desirable. ln general, the peak-topeak value of the compensating voltage less than the peak value of the erase pulse 89, Whether a bias is used on the storage screen or not. For example, the peak value of the erase pulse may be from 11/2 to 5 times the peak-to-pealt value of the compensating voltage, depending upon the particular storage tube and the conditions of operation. These relative values are merely by Way of example, and are not to be taken as limiting.

In the circuit shown in FG. l the compensating voltage wf?. and the erase pulse 89 will add in load resistor 92,. As a result, the compensating voltage itil will be superimposed on the erase pulse when it occurs. When the two voltages do superimpose, it may be desirable to synchronize them, but usually it is immaterial Whether they are synchronized or not. It may be preferred that the pealt amplitude of the erase pulse be independent of the compensating voltage. This result may be obtained by employing a non-additive combining circuit such as shown in FIG. 3. This circuit includes diodes MBS and lil-tl. The erase pulse is applied through a coupling capacitor and the diode M3 to the high potential end of load resistor 92. A high impedance resistor itlS prevents the coupling capacitor from acquiring a direct-current charge.

The compensating voltage is applied from source lill 9 brightness) as the dark screen arca is at the level shown in line M6.

For the light screen area (graph A) the light produced is represented by graph 111. The apparent brightness or light level for this area is shown by the line 117. The amount of contrast between the two light levels is indicated by the double arrow fll. A comparison of the two double arrows shows that the contrast is less `than it Was before the compensating voltage was used.

Of more importance than this comparison, however, is the fact that there are now no black areas in the picture background. The formerly black areas are at least a dark gray. This fact tends to reduce greatly the contrast because the eye is much more responsive to low level light than to higher level light. For this reason, the light level represented by line H6 appears to lthe eye to be closer t0 the level M7 than it actually is. As a result, in the eX- arnple illustrated, the compensating voltage causes a reduction in background contrast that is very substantial,

The measure of contrast may also be expressed as the ratio orr the dierence in brightness vof two regions divided by the brightness of the region considered to be the background. Expressed in this way, the contrast when using the compensating voltage is the value represented by double arrow line HS divided by the value x. Without the compensating voltage, the contrast would have a larger value equal to the value represented by double arrow line li `divided by some value much smaller than the value x.

While, for purpose of explanation, complete erasure at the end of a picture frame was assumed, it should be understood the invention applies equally Well to systems employing partial erasure such as described in connection with the circuits of FGS. l and 2. This is true whether the erasure is line by line or frame by frame.

lt should be noted that the desired reduction in blackground contrast cannot be obtained merely by reducing the amplitude of lthe erasure pulse so that no area of the screen goes black. For example, referring to lG. 5, assume Ithat the erase pulse is reduced in amplitude so that the storage screen is biased black by the voltage z, there being no compensating voltage. While the light level for the dark areas is now the same as in the example using a compensating voltage, the contrast is y/x which is la much higher value than the contrast Value obtained by using the compensating voltage.

The invention is applicable regardless of the scanning rates employed in the storage tube system. For example, the scanning rates may be those commonly employed in television. On the other hand, it may be desirable to use the storage tube for storing a picture for la long time, such as several minutes; in which case the erasure may be done under manual control. Here the invention also applies.

What is claimed is:

l. ln combination, a display storage ftube of the type having an electron permeable storage screen comprising storage elements on which a charge pattern is to be written, means for producing a writing beam for laying down said charge pattern, means including an electron source for producing a spray ct" electrons and directing them as a viewing beam toward said storage screen, a viewing screen positioned to receive any electrons or" said viewing beam that pass through said storage screen, means for erasing a charge pattern from said storage element and thereby negatively biasing said storage elements with respect to said electron source, `and means for repetitive-ly varying the potential ci said storage elements with respect to said electron source, said repetitive potential variation having a peak to peak amplitude that is substantially less than the potential to which said storage elements are biased.

2. ln combination, a display `Stor-ase tube of the type having an electron permeable storage screen comprising storage elements on which a charge pattern is -to be written, means for producing a writing beam for laying down said charge pattern, means including an electron source for producing a spray of electrons and directing them as a viewing beam `to-ward said storage screen, a viewing screen positioned to receive any electrons of said viewing beam that pass through said storage screen, means for erasing a change pattern from :said storage elements and thereby negatively biasing said storage elements with respect to said electron source, and means for rcpetitively varying the potential of said storage elements with respect to said electron source, said repetitive potential variation having a peak to peak amplitude that is substantially less than the potential to which said storage elements are biased and having a rate that is at least equal to that of persistence of vis-ion.

3. In combination, a display storage tube of the type having an electron permeable storage screen comprising storage elements on which a charge pattern is to be written, means for producing a writing beam for laying down said charge pattern, means including an electron source for producing a spray of electrons and directing themas a viewing beam toward said storage screen, a viewing screen positioned to receive any electrons of said viewing beam that pass through said .storage screen, means including an erase voltage pulse source for negatively biasing said storage elements with respect to said electron source, and means for repetitively varying `the potential of said storage elements wtih respect to said electron source, said repetitive potential variation having a peak to peak amplitude that is substantially less than the potential 'to which said storage elements are biased.

4, -ln combinaion, a display storage tube of the type having -an electron permeable storage screen comprising storage lelements cn which a charge pattern is to be written, means for producing a writing beam 4for laying down said charge pattern, meurs including an electron source for producing a spray ic elec rons and directing tl em as a viewing beam toward said storage screen, a viewing screen positioned to receive any electrons of said viewing beam that pass through said storage screen, said tube having the undesired characteristic that the viewing beam cut-oft` potential is different for some areas of the viewing screen than lor other areas, means for eras-ing a change pattern from said storage elements and thereby negatively biasing said storage elements with respect to said electron source, and means for repetitively varying the potential of said storage elements with respect to said electron source, said repetitive potential variation having a peak to peak `amplitude that is substantially than the potential to which said storage elements are biased but which is large enough to make the potential of said storage elcments alternately more negative than the maximum viewing beam cut-ofi potential for the tube `and less negative than the minimum viewing beam cuit-oil potential for the tube.

5. ln combination, a display storage tube of the type having an electron permeable storage screen comprising stor ge elements on which a charge pattern is to be writyten, means for producing a writing beam for laying down said charge pattern, means including an electron source for producing a spray of electrons and directing them` as a viewing beam toward said storage screen, a viewing screen positioned Ito receive any electrons of said viewing beam that pass through said storage screen, said tube having the undesired characteristic that the viewing beam cut-oit potential is different for some areas of ,the viewing screen than -for other areas, means for erasing a charge pattern `from said storage elements and thereby negatively iasing said storage elements with respect to said electron source, and means `lor repetitively varying the potential of .said storage elements with respect to said electron source at a rate that is at least equal to that of persistence orf vision, said repetitive potential variation having a peak to peak amplitude that is substantially less than the potential to which said storage elements are biased but which is large enough to make the viewing screen go alterless nately entirely light and entirely dark with the storage screen in the erased condition.

6. In combination, a display storage tube of the type having an electron permeable storage screen on which a clharge pattern is to be written, means for producing a Writing beam for laying down said charge pattern, means including an electron source for producing a spray of electrons and directing them as a viewing beam toward said storage screen, a phosphor screen positioned to receive any electrons ci said spray of electrons that pass through said `storage screen, said tube having the undesired characteristic that the viewing beam cut-ofi potential is diiierent for some areas of the phosphor screen than for other areas, means for producing an erase voltage pulse having a certain peak amplitude, means for applying said erase voltage pulse to said storage tube for pulsing said storage screen With respect to said electron source, and means for applying to said storage tube a repetitive voltage `for varying, the potential of said storage screen with respect to said electron source, said repetitiye vol-tage having la peak to peak amplitude that is less than the peak amplitude 'of said erase voltage pulse but which is large enough to make the phosphor screen go alternately entirely light and entirely dark with the storage screen in the erased condition.

7. ln combination, a display storage tube of the type having an electron permeable storage screen on which a charge pattern is to be Written, means for produc-ing a writing bea-m for laying down said charge pattern, means including an electron source for producing a spray of electrons and directing them as a viewing beam toward said storage screen, a phosphor screen positioned to receive viewing beam electrons that pass through said storage screen, said storage tube having the characteristic that the viewing beam cut-ori potential is a certain value for one area of the phosphor screen and is a different value for a different `area of the phosphor screen, said cut-ofi potenti-all being a maximum for one of said areas and a minimum ior another one of said areas, the voltage diierence between said maximum and `minimum cut-olf potentials being AV, means for producing an erase voltage having a certain peak amplitude, means Ifor applying said erase voltage to said storage tube for changing the potential of said storage screen with respect to said electron source, `and `means for applying to said storage tube a repetitive voltage for varying the potential of the storage elements of said storage screen with respect to said electron source, said repetitive voltage having a peak to peak amplitude that is larger than AV but substantially less than the peak amplitude of said erase voltage.

8. ln combination, a display storage tube oi' the type having an electron permeable storage screen on which a charge pattern is to be written, means for producing a writing beam for laying down said charge pattern, means including an electron source for producing a spray of elec- Cir trons and directing them as a viewing beam toward said storage screen, a phosphor screen positioned to receive viewing beam electrons that pass through said storage screen, said istorage tube having the characteristic that the viewing beam cut-oli potential is a certain value for one area of the phosphor screen `and is a different value for a diierent area of the phosphor screen, said cutaol potential being `a maximum for one of said areas and a minimum for another one olf said areas, Imeans for producing an erase voltage pulse having a peak amplitude that is intermediate said maximum and minimum cut-ott potentials, means for yapplying said erase voltage pulse to said storage tube for pulsing said storage screen with respect to said electron source, and means for applying to said storage tube a repetitive voltage 4ior varying the potential of the storage elements of said storage screen with respect to said electron source, said repetitive voltage having a peak to peak amplitude that is smaller than the peak amplitude of said erase voltage pulse but which is large enough to make the potential of said storage clements alternately more negative than the maximum viewing beam cut-o potential for the tube and less negative than the minimum viewing beam cut-oil potential for the tube.

9. A signal storage system including, an electrical storage tube having an electron permeable charge storage member, means spaced from one side of said charge storage member for providing a stream of electrons for flooding a major portion of the surface of said member, a viewing screen 'spaced from .the opposite side of said charge storage member, means for providing a sharply defined and focused beam of electrons, means for detlecting said sharply dened and focused electron beam across said storage member, connection means lfor a source of signals `for modulating said beam during said deflection to Write an electrical charge pattern on said member, the change pattern Written on said member modulating the iiow o@ said stream of electrons so that electrons passing through said electron permeable member impinge on said viewing screen and produce a visual display corresponding to said charge pattern, means coupled to said stor- Iage tube for repeti-tively generating pulses at a predetermined pulse repetition rate for periodically pulsing said charge storage member with respect to said flood beam generating means, and means for applying to said storage tube a periodic voltage of small peak amplitude as compared with the peak amplitude of said pulses for varying the potential of said charge storage member with respect to said ilood beam generating means.

References Cited in the iile of this patent UNITED STATES PATENTS 2,790,929 Herman et al. Apr. 30, 1957 2,905,849 Kazan Sept. 22, 1959 2,957,105 Taubenslag et al Oct. 18, 1960 

1. IN COMBINATION, A DISPLAY STORAGE TUBE OF THE TYPE HAVING AN ELECTRON PERMEABLE STORAGE SCREEN COMPRISING STORAGE ELEMENTS ON WHICH A CHARGE PATTERN IS TO BE WRITTEN, MEANS FOR PRODUCING A WRITING BEAM FOR LAYING DOWN SAID CHARGE PATTERN, MEANS INCLUDING AN ELECTRON SOURCE FOR PRODUCING A SPRAY OF ELECTRONS AND DIRECTING THEM AS A VIEWING BEAM TOWARD SAID STORAGE SCREEN, A VIEWING SCREEN POSITIONED TO RECEIVE ANY ELECTRONS OF SAID VIEWING BEAM THAT PASS THROUGH SAID STORAGE SCREEN, MEANS FOR ERASING A CHARGE PATTERN FROM SAID STORAGE ELEMENT AND THEREBY NEGATIVELY BIASING SAID STORAGE ELEMENTS WITH RESPECT TO SAID ELECTRON SOURCE, AND MEANS FOR REPETITIVELY VARYING THE POTENTIAL OF SAID STORAGE ELEMENTS WITH RESPECT TO SAID ELECTRON SOURCE, SAID REPETITIVE POTENTIAL VARIATION HAVING A PEAK TO PEAK AMPLITUDE THAT IS SUBSTANTIALLY LESS THAN THE POTENTIAL TO WHICH SAID STORAGE ELEMENTS ARE BIASED. 